Without limiting the scope of the invention, its background is described in connection with electronic device packaging, as an example.
Modern electronic components utilize numerous integrated circuits. These integrated circuits must often be electrically connected to each other or to other electronic components. One method for connecting integrated circuits to electronic components utilizes an area array electronic package, such as a ball-grid array (BGA) package or a flip-chip package. The electrical connections between an integrated circuit packaged in an area array package design and a printed circuit board (PCB) are typically composed of solder.
With ball grid array packages, various input and output ports of an integrated circuit are typically connected via wire bonds to contact pads of the ball grid array electronic package. Solder balls formed on the contact pads of the ball grid array electronic package are used to complete the connection to another electronic component, such as a printed circuit board (PCB).
Integrated circuits are also connected to electronic components through a flip-chip electronic package design. The flip-chip electronic package is similar to the ball grid array electronic package in that solder balls are used to make a connection with other electronic components, such as a PCB. Solder balls are also used in a flip-chip design to attach the input and output ports of the substrate to the contact pads of the integrated circuit. As such, flip-chip packages do not require wire bonds. These solder balls or bumps may be formed on the face of integrated circuits as they reside on semiconductor wafers before being sawed into individual dies.
Therefore, an important step in the interconnection of many electronic components is the formation and attachment of solder balls.
Heretofore, in this field, solder bumps or balls have been typically formed utilizing one of four methods: (1) printing of solder paste through a stencil or mask; (2) electroplating; (3) evaporation; or (4) mechanical transfer of preformed solder spheres. While electroplating, printing of solder paste through a stencil or mask, and evaporation techniques have been typically utilized for forming solder bumps on wafers and integrated circuits, BGA and chip-scale packages (CSP) have commonly utilized printing of solder paste and mechanical transfer of solder ball techniques.
Transfer of solder balls has been customarily achieved, by means of vacuum chucks or machined templates. Another method for transferring preformed solder balls utilizes formation of a pattern of dots onto a photoimageable coating laminated to an organic film. Typically the organic film is composed of a material having a high melting temperature that is capable of being exposed to temperatures exceeding 200.degree. C. with very little degradation, such as polyimide.
The pattern is formed by placing a photomask on the coating and then exposing the coating to a dose of ultraviolet radiation. For example, for an area array package design, the photomask contains a mirror image of the contact pads design. The areas protected by the photomask design retain their adhesiveness while the unprotected areas exposed to the ultraviolet radiation lose their adhesiveness. The array of adhesive areas corresponds to the pattern of contact pads found on the substrate, wafer or die to receive the solder connections.
After the adhesive areas are formed, solder balls are loaded onto the surface of the film and attach to the adhesive areas. The excess solder balls that lie on non-adhesive areas are removed. The populated film is then aligned and brought into contact with contact pads, which may be fluxed. A solder reflow is performed to transfer the solder balls from the adhesive areas to the contact pads of the substrate. Following the reflow cycle, the film is removed from the solder balls.
Before a solder sphere is reflowed to a contact pad, solder flux is usually applied to either the pad and/or the sphere to facilitate the removal of any oxides or other layers of contamination that may prevent a good sphere to pad adhesion. Solder fluxes contain organic based acids for removing the oxides. For conventional area array applications such as flip chip wafers, printed circuit boards (PCB), and ball grid arrays (BGA), the flux is; typically applied directly to the substrate by forcing the flux through a screen or stencil. For effective screening oftentimes the flux must be pasty or very viscous. A stencil is a metal mask that contains an array of holes that match the dimensions and spacing of the contact pads on the given wafer or substrate. The holes in the stencil are aligned over the contact pads.
Following the fluxing step, solder spheres are brought into contact with the fluxed contact pads and reflowed to make the electrical connection. As the center to center spacing of solder bumps becomes smaller and smaller, the ability to make cost affordable stencils becomes more difficult. Shrinking bump pitches also translates into smaller contact pads and solder spheres as well as correspondingly smaller holes in the stencil. Smaller holes places special constraints on the chemical composition and texture of fluxes in order for them to excrete through the holes and still form uniform sized droplets on the contact pads. The forces required to force the flux through small holes can also become quite excessive resulting in stencil damage.
Solder spheres formed on the contact pads of a wafer by electroplating, evaporation or printed solder paste are often sprayed with an aqueous flux solution before a reflow cycle is performed. Flux applied in this manner ends up on areas other than the contact pads of the wafer resulting in excessive flux residue. In order to provide a clean surface for subsequent processing, the excessive flux residue must be removed by a follow on clean-up process that may not be 100% effective at removing all residues.
Bumped dies sawed from the reflowed wafer are oftentimes fluxed by dipping the spheres into a thin layer of flux. The fluxed dies are then placed on a substrate and reflowed to complete the attachment. This approach will only work if the solder bumps are fairly uniform in height and if the die is flat and parallel to the flux layer ensuring that all the solder bumps make contact to the flux. The larger the die the more difficult it is to achieve and maintain a parallel surface to the flux layer. Hence, this approach does not work very well with larger die or wafers.